It is well known that some of the noise generated in machines is attributed to hydraulic noise which may be transmitted in various forms such as air borne, fluid borne, and/or structure borne. Attempts have been made in the past to control hydraulic noise by enclosing hydraulic systems in an acoustical enclosure. However, this is not feasible in many systems because some of the hydraulic components and the structures that they are mounted to are separated by significant distances. One of the primary generators of hydraulic noise in a hydraulic system is the hydraulic pump. The hydraulic pump excites fluid borne noise which is transmitted to valves, lines, and so forth and then to the structures of those components or the structures on which they are mounted. These structures then emit vibrations that create the largest portion of the overall air borne noise attributed to the hydraulic system. Therefore, reduction of fluid borne noise is a key to the reduction in the noise generated in the hydraulic system.
Positive displacement hydraulic pumps or motors, due to their geometry, port timing, and speed, inherently produce a flow ripple that excites pressure waves that are known as fluid borne noise. This is true of most, if not all, types of positive displacement vane, piston, or gear pumps or motors. For illustration purposes only, the piston pump is being used to better illustrate that which causes fluid borne noise. It is recognized that the same principles apply with respect to the other types of positive displacement pumps. The total flow output of the hydraulic piston pump is geometrically proportional to the sum of the velocities of the individual pistons between the bottom dead center (BDC) and the top dead center (TDC) positions. The uneven delivery of fluid flow resulting from the sum of the velocities not being constant is one of the inherent characteristics of a pump contributing to the flow ripple. The second source of flow ripples is due to pressure changes that occur in the piston cavity near bottom dead center when the pump is operating at some outlet pressure other than a low pressure that is equal to inlet pressure. When the piston reaches bottom dead center, the piston cavity is at inlet pressure. Until the pressure in the piston cavity reaches discharge pressure, the velocity of that piston does not contribute to the pump's output flow. Also, if the pressure in the piston cavity is not the same as the discharge pressure when the piston cavity enters the discharge port, there can be an inrush or outrush of flow between the piston cavity and the discharge cavity, causing a disturbance in the pump's output flow. The amount and rate of flow change near bottom dead center varies depending on the geometry of the cavities, the displacement of the pump, the port configuration, the pump speed and the output pressure. Thus, the flow ripple depends not only on the geometric sum of the piston velocities, but also on the pressure at which the pump is operating, the pump displacement, the pump porting, and the speed of the pump. By reducing or cancelling the flow ripple, the fluid borne noise excited by the pump is substantially reduced along with the structure borne noise and the air borne noise that are associated with hydraulic components or structures downstream thereof.
Various attempts have been made to reduce fluid borne noise in hydraulic systems by installing various mufflers and/or dampers. Likewise, port timing is sometimes changed within the pump in an attempt to modify the pressure ripple. Even though some of these attempts are proven to be partially successful, they are normally only successful when operating within narrow pressure, speed and displacement ranges of the pump. However, when systems are being operated over wide ranges of speed, displacement and pressure, these earlier arrangements have proven to be inadequate. It is desirable, therefore, to provide a system that is effective to control the fluid borne noise therein when operating at different speeds, pressures, and/or displacements.
The present invention is directed to overcoming one or more of the problems as set forth above.